Over the past twenty years, controlling the operation of pellet mills has developed from an art to a science. The assignee of the present invention has been actively involved in this development and has patented many of its developments. These patents include U.S. Pat. No. 3,932,736, believed to be the first patent issued for an automatic pellet mill controller. Further developments have also been patented including U.S. Pat. No. 4,340,937; U.S. Pat. No. 4,463,430; U.S. Pat. No. 4,671,757; U.S. Pat. No. 4,678,424; U.S. Pat. No. 4,700,310; U.S. Pat. No. 4,725,958; U.S. Pat. No. 4,729,899; U.S. Pat. No. 4,742,463; U.S. Pat. No. 4,751,030; U.S. Pat. No. 4,764,874; U.S. Pat. No. 4,929,163; and U.S. Pat. No. 4,935,874, all of which are owned by the assignee herein. Virtually all, if not all, of these patents deal with controlling and operating the pellet mill after the pellet mill has been brought up to speed. Controlling the pellet mill during "run" conditions has been considered difficult enough without factoring in an attempt to automate the start-up process. This is perhaps because of the many variables which must be monitored and accounted for during the running of the pellet mill. For example, some formulations of feed are considered to be "hard starting" while other formulations are considered easier to start as there is less risk of plugging the mill on start up. As is well known to those in the art, the starting up of a pellet mill involves setting the steam and feed inputs at relatively low levels and then slowly increasing their values to approach optimum running speeds where the mill motor is running at a significant load and the product being produced, i.e. feed pellets, is of good quality. If the start points are too aggressive, or the inputs are ramped up too rapidly, there is a risk of clogging the pellet mill such that the pellet mill must then be shut down, cleaned out, and then restarted. Obviously, this results not only in a waste of time, but significant amounts of product may be wasted as well. Thus, plugs are sought to be avoided at all costs as they can be quite expensive to a mill operator.
For these reasons, in the prior art most pellet mill controllers are arbitrarily set with initial start points at twenty percent to thirty percent of run values or the like, and no particular attention is paid to maintaining any particular ratio between the feed input and the steam input. As is known in the art, feed is input to the pellet mill by way of a screw feeder wherein the feeder speed determines the rate of input of material. Steam is input through appropriate valves, including a steam modulating valve, and is measured in terms of a steam flow rate.
In order to solve these and other problems in the prior art, the inventors herein have succeeded in designing and developing a method for determining the initial set points or start points for starting a pelleting run in a pellet mill. In essence, the automatic pellet controller already is capable of monitoring the steam flow and rate of feed input by noting the modulating steam valve position and the feeder speed as the pellet mill runs at or near its optimum condition. Generally, an optimal mill run condition includes a mill motor operating at or near its rated horsepower capacity, a temperature parameter being satisfied, and with pellets being produced of good quality as measured by standards already established in the art. By making note of these optimum run conditions, appropriate start set points may be readily calculated by multiplying these run values by an appropriately determined fixed percentage thereof. Furthermore, this process of determining start set points may be repeated each time a run of a particular formula is made. Thus, the starting set points for a particular formula may change over time due to the changing operating conditions of the pellet mill caused by wear, etc., changes in ambient temperature, changes in feed consistency or quality, changes in steam consistency or quality, as well as any other changes experienced at the actual pellet mill installation. Thus, utilizing the feature of the present invention, a pellet mill controller will continuously update and adjust the starting points for the various formulations run through the pellet mill to fine tune and ensure a rapid, trouble-free start.
There are several ways contemplated by the inventors for "capturing" the steam valve position and feeder speed corresponding to the optimum run conditions. One way would be to continuously sample these values and use the last sampled values when optimum conditions were satisfied. Another way would be to periodically sample these values, and then average the sampled values corresponding to optimum run conditions. Or, the first set of values measured, or any other mathematical weighing, could be used to arrive at the optimum run values.
Still another advantage of the present invention is that the optimal ratio between steam and feed utilized during optimal run conditions may be determined and used to set the start points such that this ratio is present in the pellet mill from the beginning of the run. This also improves the starting and transition to run speeds. Because of the successful implementation of this invention, starting set points may be used which are dramatically higher than those typically utilized in the prior art. For example, as mentioned above, typical starting set points in the prior art ranged from twenty to thirty percent of run conditions. However, it has been found by the inventors herein in implementing the present invention that starting set points of eighty percent of run conditions are not uncommon. By utilizing these elevated start set points, the pellet mill is able to reach a final run condition much more rapidly and the product produced during the start-up phase is of much better quality and closer to that produced by the pellet mill in its full run mode. Thus, tremendous savings are realized not only in mill run time required to complete a typical run, but also wasted material is dramatically decreased.
In implementing this automated start-up methodology, the inventors have also addressed the problems induced in controlling the pellet mill with a modulating steam valve that is non-linear. In other words, with the kinds of ball valves generally used for modulating steam flow, there is a non-linear relationship between the opening of the ball valve and the amount of steam flow therethrough. For example, when the modulating steam valve is fifty percent open, the permitted steam flow therethrough is significantly less than fifty percent. Thus, the digital control of the modulating steam valve typically utilized in computerized automated pellet mill controllers, while effective in knowing what percentage the steam valve was open, was mistakenly using that percentage as corresponding to the same percentage of steam flow therethrough. In implementing the present invention, that difference became significant and the inventors herein have.succeeded in implementing a methodology for eliminating that non-linear offset by determining the appropriate steam flow at run conditions, calculating an appropriate starting steam flow, and then converting that starting steam flow to a corresponding steam valve position. This inventive feature is also helpful in the actual control of the pellet mill itself in that, for the first time, actual steam flow through the steam valve may be determined by the controller and utilized instead of the steam valve position which did not correspond to steam flow in a linear fashion.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.